Recently, social need for weight saving of vehicles such as motorcars has increased more and more out of consideration for the global environment. To meet such social need, as a material of an auto panel, particularly a large body panel (an outer panel and an inner panel) such as a hood, a door, and a roof, a more lightweight aluminum alloy material having excellent formability and paint-bake hardenability is increasingly used in place of steel materials such as steel sheets.
In particular, Al—Mg—Si-based aluminum alloy sheets such as AA-series or JIS6000-series, which may be simply referred to as 6000-series below, are used as thin and high-strength aluminum alloy sheets for panels including an outer panel and an inner panel of a panel structure such as a hood, a fender, a door, a roof, and a trunk lid of a motorcar.
The 6000-series (Al—Mg—Si-based) aluminum alloy sheet essentially includes Si and Mg. In particular, an excessive-Si-type 6000-series aluminum alloy has a composition where such Si and Mg satisfy Si/Mg of 1 or more in mass ratio, and exhibits excellent artificial age hardenability after forced heating. The aluminum alloy sheet therefore has paint-bake hardenability, which may be referred to as bake hardenability (=BH property) or baking hardenability below, that allows formability during press forming or bending to be ensured by lowered proof stress, and allows strength necessary for the formed panel to be ensured by increased proof stress due to artificial age hardening through forced heating during artificial aging (hardening) at relatively low temperature such as paint baking treatment of a formed panel.
Moreover, the 6000-series aluminum alloy sheet has a relatively small amount of alloy elements compared with other aluminum alloys such as 5000-series aluminum alloy having a large alloy amount such as Mg amount. Hence, when scrap of such a 6000-series aluminum alloy sheet is reused as an aluminum alloy melting material (melting source material), an original 6000-series aluminum alloy slab is easily reproduced, showing excellent recyclability of the 6000-series aluminum alloy sheet.
On the other hand, as well known, an outer panel of a motorcar is fabricated through various types of forming, such as stretch forming as a type of press forming and bending, performed on an aluminum alloy sheet. For example, in fabrication of a large outer panel such as a hood and a door, the aluminum alloy sheet is formed into a product shape of the outer panel by press forming such as stretch forming, and then the formed product is joined to an inner panel through hemming such as flat hem of the periphery of the outer panel, so that a panel structure is formed.
The outer panels of the motorcars tend to be reduced in thickness for weight saving, and are required to have higher strength so as to have excellent dent resistance despite the reduced thickness. Hence, the aluminum alloy sheet is further required to have the artificial age hardenability (paint-bake hardenability) that allows formability to be secured by lowered proof stress of the aluminum alloy sheet during press forming, and allows necessary strength to be secured even after thickness reduction by increased proof stress through age hardening caused by heating during artificial aging at relatively low temperature, such as paint baking of a formed panel.
It has been variously proposed that an Mg—Si-based cluster, which is formed in the 6000-series aluminum alloy sheet left at a room temperature after solution and hardening, is controlled for such paint-bake hardenability of the 6000-series aluminum alloy sheet. Each of such proposals mainly improves paint-bake hardenability by heat treatment, etc. after solution and hardening in fabrication of the aluminum alloy sheet. In a recently proposed technique, such an Mg—Si-based cluster is controlled after being measured with an endothermic peak and an exothermic peak on a differential scanning calorimetry curve, which may be referred to as DSC below, of the 6000-series aluminum alloy sheet.
For example, PTL 1 and PTL 2 each propose limiting production of such an Mg—Si-based cluster, particularly a Si/vacancy cluster (GPI), as a factor impairing the low-temperature age hardenability. In such techniques, it is defined that no endothermic peak exists in a temperature range from 150 to 250° C. corresponding to melt of GPI on DSC of T4 material (subjected to natural aging after solution) in order to limit production of GPI that impairs suppression of room-temperature aging and the low-temperature age hardenability. Furthermore, in such techniques, the aluminum alloy sheet is subjected to low-temperature heat treatment, i.e., held at 70 to 150° C. for about 0.5 to 50 hr after solution and hardening down to room temperature in order to suppress or control production of GPI.
As described in PTL 1 and PTL 2, GPI, which is formed during room-temperature after solution and hardening, is collapsed at paint baking, and solute concentration of a matrix is lowered, and therefore precipitation of a GP zone (Mg2Si precipitated phase) contributing to increase in strength is hindered, and thus the low-temperature age hardenability is impaired. Furthermore, formation of the GPI increases strength, and impairs suppression of room-temperature aging. Hence, suppressing formation of GPI improves the suppression of room-temperature aging and the low-temperature age hardenability. However, only suppressing formation of the GPI is not enough for the recently required improvement of paint-bake hardenability (low-temperature age hardenability). For example, while PTL 1 and PTL 2 each disclose the paint-bake hardenability, proof stress after BH under an artificial aging condition of 175° C.×30 min or 170° C.×20 min is at a level of about 168 MPa at a maximum, which does not satisfy 200 MPa or more required for this type of panel application.
PTL 3 proposes an excessive-Si-type 6000-series aluminum alloy material satisfying that height of a minus endothermic peak is 1000 μW or less in a temperature range from 150 to 250° C. corresponding to dissolving of a Si/vacancy cluster (GPI), and height of a plus exothermic peak is 2000 μW or less in a temperature range from 250 to 300° C. corresponding to precipitation of a Mg/Si cluster (GPII) on DSC of this aluminum alloy material subjected to tempering including solution and hardening. This aluminum alloy material, which is subjected to the above-described tempering and then subjected to room-temperature aging for at least four months, has the following properties: proof stress is within a range from 110 to 160 MPa, a difference in proof stress with respect to the aluminum alloy material immediately after the tempering is within 15 MPa, elongation is 28% or more, and proof stress is 180 MPa or more after low-temperature aging of 150° C.×20 min after application of strain of 2%.
However, such a technique of PTL 3 is also less likely to control an aluminum alloy sheet, of which the As proof stress immediately after tempering (fabrication) is less than 135 MPa, to have high proof stress, i.e., have proof stress after BH after paint-bake hardening (under a condition of 170° C.×20 min after application of strain of 2%) of nearly 240 MPa or more. In other words, the aluminum alloy sheet is less likely to have a paint-bake hardening property (BH property) ensuring a difference of 120 MPa or more between the proof stress after BH and the As proof stress.
In PTL 4, to attain the BH property after the paint-bake hardening under such a condition of low temperature and short time, it is defined that exothermic peak height W1 is 50 μW or more in a temperature range from 100 to 200° C., and a ratio of exothermic peak height W2 in a temperature range from 200 to 300° C. to the exothermic peak height W1 W2/W1 is 20.0 or less on a differential scanning calorimetry curve of the 6000-series aluminum alloy sheet subjected to tempering.
The exothermic peak W1 corresponds to precipitation of the GP zone to be a nucleation site of β″ (a Mg2Si phase) during artificial age hardening, and as the peak height of W1 is higher, a larger amount of GP zone to be a nucleation site of β″ during artificial age hardening is already formed in the tempered aluminum alloy sheet subjected. As a result, β″ is promptly grown during paint-bake hardening after forming, so that paint-bake hardenability (artificial age hardenability) is improved. On the other hand, the exothermic peak W2 corresponds to a precipitation peak of β″ itself, and height of the exothermic peak W2 is controlled to be as low as possible in order to lower the proof stress of the tempered (fabricated) aluminum alloy sheet to less than 135 MPa to ensure formability.